When idling an engine, in general, the amount of intake air supplied to the engine is adjusted to control an idling speed close to a target speed, thereby improving a fuel consumption of the engine. Typically, to regulate the intake air amount during idling, the opening degree of a bypass valve, provided in a bypass passage bypassing a throttle valve and having both ends connected to an intake passage, is adjusted in accordance with a difference between a detected value of the engine speed and a target idling speed. This thereby increases or decreases the amount of air (intake air amount) supplied to the engine through the bypass passage so as to suppress fluctuations in the engine speed, thus ensuring stable idling.
A further improvement of fuel consumption of the engine can be achieved by reducing idling speed. Especially when driving in an urban area where the engine runs at an idling speed more frequently, the effect of the improved fuel consumption is significantly higher. Idling at a lower speed, however, causes the engine speed to easily fluctuate because of a deteriorated combustion condition due to an increase in an internal EGR amount, increased friction in the engine, and deteriorated stability of a loop transfer system of intake air--engine output torque--engine speed, so that stability of idling is badly affected. Especially, if the intake system of an MPI (Multi-Point Injection) type engine wherein fuel is injected and supplied from fuel injection valves provided for respective cylinders includes a large-volume surge tank, the stability of the loop transfer system and hence idling stability are deteriorated because of the larger volume of the intake system.
An L-jetronic type control system which employs an air flow sensor is known as an engine control system featuring highly stable idling. In a typical L-jetronic system, the amount of air passing through the throttle valve per cylinder stroke is directly measured by the air flow sensor, as the actual amount of intake air supplied to a cylinder, and an amount of fuel according to the measurement result is injected. Thus, when the engine speed (repetition of intake per unit time) decreases, the amount of fuel supplied to one cylinder increases in inverse proportion to the decreased amount in engine speed, so that an air-fuel ratio is made richer. Accordingly, the L-jetronic system exhibits good controllability as long as the engine is subjected to idling under a specific condition.
Actually, however, even when the engine is running in a steady idling condition, fluctuations in combustion may take place, or minute fluctuations may occur in rotational speed due to a manufacturing error of a crank angle sensor or the like. The L-jetronic system has a high air-fuel ratio modulation working gain, therefore, the working gain may be excessive when the aforesaid fluctuations happen. In this case, an excessive change in the air-fuel ratio occurs, thus resulting in unstabilized idling. To avoid such inconvenience, the control gain in controlling the air-fuel ratio of the engine mounted on a commercially available vehicle must be reduced, whereby the controllability is accordingly deteriorated. Additionally, if no limitation on the manipulating amount or manipulating signal associated with the air-fuel ratio is provided in the L-jetronic system, then the air-fuel ratio tends to be set to an excessively large value (overlean) when the engine speed suddenly increases. This may cause misfire.
Further, a technique for stabilizing idling for a D-jetronic type engine, which employs a sensor for detecting a pressure in an inlet pipe, has been proposed ("Automotive Technology" Vol. 37, No. 9, 1983, pp. 986-991). In this method, an actual negative pressure in the inlet pipe in a few strokes ahead is predicted based on a variation (differential value) in the negative pressure in the inlet pipe per stroke, and the fuel injection amount is calculated in accordance with the predicted value, whereby the delay or time lag in detection of the negative pressure in the inlet pipe is compensated, to improve the stability of idling. This proposed method, which allows a dead zone or a rich/lean upper value to be set as necessary, has flexibility that makes it possible to avoid the danger of misfire or the like found in the L-jetronic method described above. In practical use, however, this method presents a problem in that the differential values of the information on the negative pressure in the inlet pipe cannot be detected with high accuracy because of intake pulsation. Further, the proposed method presents a problem in that, if the differential gain is set to a large value so as to obtain a sufficient detection delay compensation effect, then considerations must be given even to the influences exerted by an A/D conversion error on the information on the negative pressure in the inlet pipe.
Further, when driving at an extremely slow speed with use of a lower gear position, the engine speed and load are close to the idling region, so that a periodical rotation fluctuation (below 1 Hz), similar to idle hunting, may take place occasionally. When it occurs, a vehicle generates an uncomfortable longitudinal shake called a surge. This is referred to as a crawling surge. This phenomenon is considered to be caused by a vibration developed from the first order delay in the intake system, which corresponds to the fluctuation in the idling revolution and is amplified by the torsional resonance of a driving system.
To solve problems of this kind, the present inventor has proposed a method for stabilizing idling (unexamined Japanese patent publication no. H4 50446), which method is based on the information on engine speed which is highly stable to be used as a control information with lower noises or fluctuations compared to the information on the negative pressure in the inlet pipe. According to this proposed method, an instantaneous value of the information on the engine speed is determined, and the instantaneous value is subjected to first order delay processing. Then, an ignition timing is delayed or advanced and fuel supply amount is increased or decreased in accordance with a difference between the instantaneous value and the value obtained by the first delay processing. However, a technological concept for optimally associating the difference with the correction amounts (more generally, associating the detected information with the manipulating amount for engine control) was not entirely clear.